ABSTRACT/SUMMARY Throughout the CNS diversity in the location and laminar organization of subpopulations of neurons and their synaptic circuits underlies functional specificity, as is evident in the visual pathways mediating low versus high resolution images. The degree of circuit specification in the olfactory bulb remains controversial. Functional analyses suggest a topography of odor-induced activity, but this has focused on the glomeruli where sensory neuron axons expressing the same odor receptor converge and terminate; conflicting data has been reported on the odor-specificity of mitral cells and the deeper radial and horizontal circuits in the bulb. Our proposal has two primary goals. First, there is little information on the innervation of individual glomeruli by subsets of mitral cells. Estimates suggest that each glomerulus is innervated by a subset of 25 mitral cells, but given the heterogeneity in the volume of individual glomeruli, this is likely a misleading generalization. Whether the mitral cells innervating single glomeruli share a common birth date, migratory timeframe or molecular phenotype are also not known. Second, while several studies have examined deafferentation in the bulb, we know comparatively little about trophic mechanisms influencing the fine structural organization of dendrites and their synaptic organization. In brief, our overarching goal is a better understanding of the embryonic and perinatal events underlying the primary cellular organization in the olfactory bulb. To achieve this goal our specific aims are summarized as: 1) Determine the principles underlying targeting of glomeruli by subsets of mitral cells; 2) Establish the relationship between birth date, apoptotic cell death, and topography of olfactory bulb mitral cells; and 3) Test the hypothesis that trophic mechanisms, in particular BDNF, contribute to the differentiation and maturation of mitral cells. This work provides insight into determinants of early development relevant to diseases such as Kallman's syndrome and autism; the principles derived are also likely relevant to broad categories of anomalous cortical development and syndromes such as Fragile-X.